Pump the heart rate up, then down, up, down . . .

Working out at different intensity levels keeps things fresh and the body from hitting a plateau.

The words "interval training" can strike fear in the hearts of even the most athletic men and women. Alternating periods of high-intensity work with recovery may sound simple enough, but those intense bouts can leave the exerciser gasping for air.

It's that intensity that ultimately reaps great benefits, including strengthening the heart and improving the cardiovascular system, which is why personal trainers tend to favor interval training. Higher exercise levels can be achieved by upping the speed or the resistance -- think walking faster on a treadmill, increasing the incline, or both.

Intervals can be done on cardio equipment such as stationary bikes and elliptical trainers, and runners can toggle between slower speeds and sprints. Strength-training circuit workouts can even incorporate them by adding full-body exercises.

Here, three trainers explain how to integrate interval workouts into an exercise plan.

--

The basics of interval training are tough to figure out without doing some math first to figure out one's maximum heart rate.

A general formula for calculating that number is 220 minus age. To work in a lower aerobic zone, in which the body burns primarily fat for fuel, calculate 65% of the maximum heart rate. To work in a higher anaerobic zone, calculate 85% of the maximum heart rate. The anaerobic zone, in which the body burns primarily carbohydrates for fuel, is the high-intensity portion of the interval (some trainers may put it at about 80%). For those who like precise numbers, some gyms and training facilities offer more technically advanced testing, but that can be costly. Heart rates can be taken manually at the wrist, and some cardio machines have built-in monitors (although accuracy varies). Heart-rate monitors provide the best readings.

Tony Ambler-Wright

Scottsdale, Ariz.-based master instructor for the National Academy of Sports Medicine:

We take our clients in and out of three zones: Zone 1 is 65% to 75% of your maximum heart rate, and Zone 2 is 80% to 85% of your maximum heart rate. Zone 3 is 85% to 90% of your maximum heart rate, and you usually don't want that to comprise more than 10% of your total training volume for the week because it's so intense. But ultimately the goal is to get everyone to that point where they could exercise at that level of intensity.

You should be able to sustain the upper level of Zone 1 for 30 minutes before trying to do some interval training. If you're able to do half an hour to 45 minutes in zones 1 and 2, you're a good candidate to incorporate some Zone 3 work in small intervals, within a workout that utilizes zones 1 and 2.

The work-to-rest ratio can change. You can go up into Zone 2 for three minutes, then down to Zone 1 for three minutes. Then you can work a 2-1 ratio of work and rest. A good rule of thumb is -- and this is where the heart-rate monitor comes in handy -- that when you drop down a zone, you should see a 20-beat-per-minute drop in your heart rate within one minute. That's a good indicator that you're in the recovery zone.

People have a tendency to push themselves too hard, too soon. A lot of people don't do the preparatory work in getting the body used to exercising and improving their base, where the body is utilizing fat efficiently as a fuel source, before doing intervals.

If people have a structured workout, then they have to get in and out of their heart-rate zones at set times. Because they're so focused on adjusting the time and intensity, they lose track of how much time they're spending exercising. Most people say, "Wow, the time really flew by."

Brian Cinadr

Personal trainer and co-owner of Canyon Athletics in Santa Monica:

Interval training is sort of an exaggerated level of fitness, and it allows you to train hard, without training hard the whole time. Doing interval training is in some ways less burdensome than the idea of going out and running for an hour. That's a grind. This is up and down, which keeps it a bit fresh.

There are other ways of doing it [rather than just cardio]. You can walk, then stop and do a set of squats or push-ups. You're going to get some body shaping and strength and flexibility benefits, and you're going to raise your heart rate more than just by walking because you're pumping blood to your upper body.

There are no rules for interval training. For a beginner, even a minute or two at 85% of your maximum heart rate is pretty intense. You want to keep it fun too. The biggest mistake people make is they get motivated right away, then completely destroy that motivation because they're so worn out. Just the fact that you're doing something is great.